To effectively code speech sounds, CELP (“Code Excited Linear Prediction”) type techniques are advocated. On the other hand, to effectively code musical sounds, transform coding techniques are advocated.
Coders of CELP type are predictive coders. Their aim is to model the production of speech on the basis of various elements: a long-term prediction for modeling the vibration of the vocal chords in a voiced period, a stochastic excitation (white noise, algebraic excitation), and a short-term prediction for modeling the modifications of the vocal tract.
Transform coders use critical sampling transforms to compact the signal in the transformed domain. A transform for which the number of coefficients in the transformed domain is equal to the number of coefficients of the digitized sound is called a “critical sampling transform”.
One solution for effectively coding a signal containing these two types of content consists in selecting in the course of time the best technique. This solution has in particular been advocated by the 3GPP (“3rd Generation Partnership Project”) standardization body, and a technique named AMR WB+ has been proposed.
This technique is based on a CELP technology of AMR WB type and a transformation coding based on an overlap Fourier transform.
This solution suffers from inadequate quality in the music. This inadequacy stems particularly from the transform coding. Indeed, the overlap Fourier transform is not a critical sampling transformation, and therefore, it is sub-optimal.
Moreover, the windows used in this coder are not optimal in regard to energy concentration: the frequency forms of these windows are relatively frozen.
Critical sampling transformations are known. For example, the transforms used in the music coders of MP3 and AAC type. These transforms rely on the formalism called TDAC (“Time Domain Aliasing Cancellation”).
The use of TDAC makes it possible to obtain excellent quality in the music. Nonetheless, this has the drawback of introducing temporal aliasings which hinder combination with technologies of CELP type.
Indeed, during a transition of TDAC to CELP type the temporal aliasing of the TDAC part is not canceled by the signal arising from the CELP, the latter not incorporating any aliasing.